This invention relates to the use of N-(aminoalkyl)pyrrolidines as catalysts for the preparation of products by the polyisocyanate addition process. These catalysts may be used as a replacement for or in combination with known urethane catalysts, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), for the preparation of rigid or flexible polyurethane foams and other polyurethane products. In the context of the present invention, the term "polyurethane products" is intended to encompass all reaction products of polyisocyanates with compounds containing at least two isocyanate-reactive hydrogen atoms. That is, the term "polyurethane" is understood to encompass, for example, pure polyurethanes, polyurethane polyureas, or pure polyureas.
The rate of the reaction between isocyanate groups and compounds containing NCO-reactive hydrogen atoms is determined not only by the temperature of the starting products and their structure but particularly by the use of suitable catalysts. In practice, bases (for example, tertiary amines such as triethylamine) are used mainly as nucleophilic catalysts, whereas organometallic compounds (for example, tin carboxylates such as tin(II) octoate) are used mainly as electrophilic catalysts. The prior art processes are based on the joint use of Lewis acids and Lewis bases, which is normally characterized by synergistic effects. However, it is also known that amines are exclusively used as catalysts in a number of applications. However, only a few of the large number of known amine catalysts (cf. Ullmann, 4th Edition, and Kunststoffhandbuch, Vol. VII, Polyurethane, Hanser-Verlag, Munich (1983)) have hitherto been adopted for use on a wide scale, with 1,4-diazabicyclo[2.2.2]octane (DABCO), bis(2-dimethylaminoethyl) ether, triethylamine, dimethyl cyclohexylamine, dimethylethanolamine, dimethylbenzylamine, methylmorpholine, and ethylmorpholine being the most important. More particularly, of course, catalysts distinguished by high activity, economic production, and a broad range of applications are used. Another increasingly important consideration is the toxicological evaluation of the catalysts with regard to processing safety and odor emission. Many of the amine catalysts in use today, such as DABCO or triethylamine, may be regarded as unsatisfactory in this respect because of their high volatility and the relatively intensive amine odor that is carried over into the end product produced with such catalysts.
Amine catalysts containing an additional, isocyanate reactive group corresponding to the general formula ##STR2## are described in German Offenlegungsschrift 2,116,535, which also mentions, inter alia, compounds in which the two substituents R are attached to each other to form 3- to 6-membered rings. The use of one representative member of this special class of compounds, namely N-(2-aminoethyl)aziridine, is illustrated by Examples 10 and 16-18 of the German patent. In the Examples mentioned, this compound proved to be distinctly poorer than the corresponding acyclic compounds with regard to the density, strength, and elasticity of the foams prepared (Example 10) and particularly with regard to activity (Examples 16 to 18).
It has now surprisingly been found that certain pyrrolidine derivatives may be used advantageously as catalysts for the preparation of polyurethanes. Compared with the above-mentioned N-(2-aminoethyl)aziridine and cyclic compounds other than those having 5-membered rings, the compounds used in accordance with the invention have considerably greater activity that even surpasses that of the acyclic members disclosed in the above-cited German patent specification. Another advantage is the faint odor and low volatility of compounds in which the isocyanate-reactive groups are bound in the polymer, which leads to distinctly reduced odor emission in the preparation of polyurethane products. In addition, other advantages have been observed, including, for example, ease of handling (because the pyrrolidines preferably used are liquid), good curing behavior, and, not least, the very simple preparation of the compounds.